1. Field
The field is pad printing, and in particular rotary transfer pad printing.
2. Prior Art—FIGS. 1 through 3
FIGS. 1 and 2 show a prior-art rotary pad printing system. FIG. 3 shows an elliptically-shaped object printed on such a system.
In the past, rotary pad printing has been used to decorate objects by printing images or text thereon. In its simplest form, a rotary pad printing apparatus comprises a rotary cliche ink-image donor roll 100 (FIG. 1), a compressible rubber transfer pad in the form of a wheel 105 typically formed around a steel shaft 106, and an ink source 110. An object to be decorated 115 is arranged to move in contact with wheel 105.
An image is first etched into rotary cliche 100, in well-known fashion. In this case, the numbers 1 through 5 are etched in cliche 100 to a depth of approximately 0.03 mm. Although numbers are shown here, the image can comprise text, graphics, and even photographic information.
Next, cliché 100 is placed into the printing apparatus, as shown in FIGS. 1 and 2. Ink supply 110 is filled with ink prior to printing. When printing commences, cliche 100 and wheel 105 are driven to rotate against one-another by a mechanism (not shown). Directions of motion are indicated by arrows. As cliché 100 turns, ink from source 110 is doctored into the etched surface of cliché 100 by ink source 110, in well-known fashion.
As cliché 100 turns against wheel 105 (FIG. 2), image-wise regions of ink 120 are transferred in near-entirety from cliché 100 to wheel 105. As rotation of cliché 100 and wheel 105 continues, ink regions 120 on wheel 105 are brought into contact with object 115. Object 115 is moved linearly as wheel 105 rotates. There is no slippage between the surfaces of object 115 and wheel 105. As ink regions 120 come into contact with object 115, they leave wheel 105 and most of ink in regions 120 transfers to object 115. Thus the image originally present on cliché 100 is transferred to object 115. Instead of using a flat object 115 and moving it linearly, an object with round or ellipsoid cross-section can be rotated against wheel 105 with axes of the object and the wheel parallel. This type of transfer motion is shown below.
Single-color images are transferred in the manner described above. If the user wishes to transfer multi-color images, color separations are required and a separate cliché is required for each color. Making a separate cliché for each color is expensive and time-consuming. Mounting and aligning separate cliché is also time-consuming. The objects to be decorated must be carefully aligned for subsequent passages through the rotary transfer pad printing apparatus. In general, this prior-art arrangement is suitable only for single-color transfers.
Since the image on the cliché is etched, each cliché contains only one image. Changing the image requires etching a new cliché and exchanging the new cliché for the old one. Thus it is impractical to use the prior-art apparatus to print small runs.
Furthermore, since the cliché transfers the etched and inked image to the rotary pad, the length of the image that can be transferred is limited by the circumference of the cliché.